The structure of expanded PTFE (“ePTFE”) is well known to be characterized by nodes interconnected by fibrils, as taught in U.S. Pat. Nos. 3,953,566 and 4,187,390, to Gore, and which patents have been the foundation for a significant body of work directed to ePTFE materials. The node and fibril character of the ePTFE structure has been modified in many ways since it was first described in these patents. For example, highly expanded materials, as in the case of high strength fibers, can exhibit exceedingly long fibrils and relatively small nodes. Other process conditions can yield articles, for example, with nodes that extend through the thickness of the article.
Surface treatment of ePTFE structure has also been carried out by a variety of techniques in order to modify the ePTFE structure. Okita (U.S. Pat. No. 4,208,745) teaches exposing the outer surface of an ePTFE tube, specifically a vascular prosthesis, to a more severe (i.e., higher) thermal treatment than the inner surface in order to effect a finer structure on the inside than on the outside of the tube. One of ordinary skill in the art will recognize that Okita's process is consistent with prior art amorphous locking processes, the only difference being preferential exposure of the outer surface of the ePTFE structure to greater thermal energy.
Zukowski (U.S. Pat. No. 5,462,781) teaches employing plasma treatment to effect removal of fibrils from the surface of porous ePTFE in order to achieve a structure with freestanding nodes on the surface which are not interconnected by fibrils. No further treatment after the plasma treatment is disclosed or contemplated in the teachings.
Martakos et al. (U.S. Pat. No. 6,573,311) teach plasma glow discharge treatment, which includes plasma etching, of polymer articles at various stages during the polymer resin processing. Martakos et al. distinguish over conventional processes by noting that the prior art techniques operate on finished, fabricated and/or finally processed materials, which are “ineffective at modifying bulk substrate properties, such as porosity and permeability.” Martakos et al. teach plasma treating at six possible polymer resin process steps; however, no such treatment with or subsequent to amorphous locking is described or suggested. Again, Martakos et al. is directed to affecting bulk properties such as porosity and/or chemistry quality in the finished articles.
Other means of creating new surfaces on porous PTFE and treating the surface of porous PTFE abound in the prior art. Butters (U.S. Pat. No. 5,296,292) teaches a fishing flyline consisting of a core with a porous PTFE cover that can be modified to improve abrasion resistance. Abrasion resistance of the flyline is improved by modifying the outer cover either through adding a coating of abrasion resistant material or by densifying the porous PTFE cover.
Campbell et al. (U.S. Pat. No. 5,747,128) teach a means of creating regions of high and low bulk density throughout a porous PTFE article. Additionally, Kowligi et al. (U.S. Pat. No. 5,466,509) teach impressing a pattern onto an ePTFE surface, and Seiler et al. (U.S. Pat. No. 4,647,416) teach scoring PTFE tubes during fabrication in order to create external ribs.
Lutz et al. (US 2006/0047311 A1) teach unique PTFE structures comprising islands of PTFE extending from an underlying expanded PTFE structure and methods for making such structures.
None of these documents teaches a uniquely stabilized PTFE fabric or laminate structure.
For numerous conventional applications, including filtration, garments. etc., fabrics are bonded to membranes in order to reinforce them. The fabrics provide handleability and structural stability to otherwise relatively delicate membranes. PTFE fabrics offer unique advantages which include, but are not limited to, chemical inertness and extreme operating temperature range. Fabrics comprising expanded PTFE offer the further advantage of increased strength compared to non-expanded PTFE fabrics.
PTFE-based fabrics are inherently difficult to bond to membranes, and accordingly, the bonds tend to be weak. For applications demanding the benefits of PTFE or ePTFE fabric reinforcement, thermal bonding techniques, with or without the use of adhesives, are typically used to bond the fabric to the membrane. Since adhesives do not exhibit the same inertness or operating temperature range of PTFE or ePTFE, they tend to compromise the performance of the resultant laminate during use. Additionally, limitations in bond strengths of conventional adhesives, such as FEP and PFA and the like, can compromise product performance in such demanding applications as fluid filtration. Adhesives can also flow onto the membrane surface during the bonding process, thereby compromising membrane performance. For instance, in the case of filtration membranes, excess adhesive can inhibit flow through the affected portion of the membrane, thereby decreasing liquid or gas filtration effectiveness.
When the membrane to be bonded also comprises PTFE or ePTFE, achieving effective bonding can present even greater difficulty. EP 1094887 B1, to Griffin, and U.S. Pat. No. 4,983,434, to Sassa et al., teach examples of filtration products wherein fabrics comprising PTFE are bonded with adhesive to ePTFE membranes.
A long felt need has existed for laminates comprising PTFE fabric-reinforced membranes with enhanced peel strength.